Galactic halo subdwarfs and white dwarfs are locally very scarce and many of their characteristics are hence poorly understood. As the most common members of the spheroid, however, they are crucial to the understanding of our own and other galaxies, able to yield key information about the shape, formation, chemical history and dark matter of the spheroid, as well as providing clues about the processes of stellar evolution.
Wide-field photographic data spanning observations taken over long time baselines, such as those available from the SuperCOSMOS Sky Survey (SSS), are unparalleled in their ability to identify large numbers of these dwarf spheroid stars through their large space motions. However, the “Achilles Heel” of photographic astronomy in studies such as this is poor photometry: a problem which can now be circumvented - whilst retaining the astrometric information of the photographic data - with the advent of large-scale, deep CCD surveys with accurate photometry such as the Sloan Digital Sky Survey (SDSS). In this thesis I show that the combination of these two types of dataset brings vast numbers of locally-rare dwarf spheroid stars into the observational reach of astronomers, yielding reliable samples many times larger than have previously been available solely from photographic data.
Using SSS data coupled with the SDSS archive I identify a sample of ~2600 candidate subdwarfs through strict selection criteria. This forms one of the largest and most reliable samples of subdwarfs known, and enables accurate determination of luminosity functions along many different lines of sight. I derive the subdwarf luminosity function with unprecedented accuracy to M y £ 12.5, finding good agreement with recent local estimates but discrepancy with results for the more distant spheroid. This provides further evidence that the inner and outer parts of the stellar halo cannot be described by a single density distribution. I also use the data to show that the form of the inner spheroid density profile within distances of 2.5 kpc is closely matched by a power law with an index of a = —3.15 ± 0.3. Whilst this study is unable to provide further constraints on Galactic structure at present, development of these methods and results have the potential to yield much more information on the formation and evolution of the Galaxy.